As is well-known, in a series electric motor, the armature and the field coils are connected in series. In addition, commutating field coils (interpoles), which are connected in series with the armature and the series field coils are sometimes utilized to neutralize the reactive voltage induced in the armature when undergoing commutation. In this manner, brush sparking is minimized. However, if the commutating field and the series field suddenly collapse due to a power failure, the self-inductance of the commutating and the series field coils produces a very large voltage surge which must be dissipated by the motor insulation. This voltage surge may cause arcing from the brush holder to the commutator or to the motor frame, and commutator flashover. The end result is that the motor insulation will be weakened, the commutator might be damaged by the flashover, and an unsafe condition might be created by the arcing.
The foregoing problems have been minimized in shunt motors by the use of a shunt field coil discharge resistor, however, such resistors are typically placed within the motor controller rather than within the motor. The purpose of such resistors is to protect the controller from contactor arcing rather than to protect the motor, and thus provide no protection for the motor insulation, the commutator, and the brush holders.
Because of the foregoing, it has become desirable to develop a device for protecting the components of a DC series motor from the voltage surge which occurs as a result of the collapse of the motor fields.